This invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof. These devices are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy, laser angioplasty or other means.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as a coronary artery. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway therethrough.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from expandable heat sensitive metals; self expanding stents inserted in a compressed state for deployment in a body lumen, and stents shaped in zig zag patterns. One of the difficulties encountered using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the patient""s vasculature. Generally, the greater the longitudinal flexibility of the stent, the easier and more safely it can be delivered to the implantation site.
Various means have been described to deliver and implant stents. One method frequently described for delivering of a stent to a desired intraluminal location includes mounting the stent on an expandable member, such as a balloon, provided on a distal end of an intravascular catheter, advancing the catheter to the desired location within the patient""s body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition, and then deflating the balloon and removing the catheter. Other prior art stent delivery catheters used for implanting self-expanding stents include an inner member upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is placed over the compressed stent to maintain it in its compressed state prior to deployment. When the stent is to be deployed in the body vessel, the outer restraining sheath is retracted in relation to the inner lumen to uncover the compressed stent, allowing the stent to move into its expanded condition.
Stents can be formed from metal alloy tubing, such as stainless steel, along with other biocompatible materials and metal alloys including, but not limited to tantalum and NiTi. Such stents can be made in a number of different ways. One method is to cut a thin wall tubular member to remove portions of the tubing in a desired pattern for the stent, leaving a relatively untouched portions of the metallic tubing which cooperate to form the stent. Machine-controlled lasers are but just one method for cutting the tubing into the desired pattern. Other methods include chemical etchings which remove the portions of the tubing leaving the untouched portions to form the desired pattern for the stent. Still other methods include bending coiled wires in the desired pattern to create the composite stent. Such techniques may include the need to weld and braze coils together to create the composite stent. Such coiled wire stents are often labor-intensive and difficult to achieve a finished product.
The stent structure may be coated with biocompatible coatings to help prevent the body from rejecting the implant. Therapeutic drugs are sometimes coated on the stent surface and are absorbable in the area of treatment over a period of time to help prevent restinosis and to help prevent body rejection of the stent.
It will be apparent from the foregoing that conventional stents are very high precision and, ideally, the most desirable stents usually incorporate a fine precision structure. In this regard, it is important to make precisely dimensioned, smooth, stents in fine geometries without damaging the narrow struts that make up the stent structure. While various cutting processes, including laser cutting and chemical etching have been adequate, improvements have been sought to provide stents of enhanced structural quality at reduced cost.
Accordingly, those concerned with the development, manufacture and use of stents have long recognized the need for improved manufacturing processes for making such stents. The present invention fulfills these and other needs.
The present invention provides a new and improved method for making a stent. In accordance with the present invention, it is preferred to form the stent using a rotational molding or centrifugal casting process. The present invention also is directed to a vascular stent formed from such a process.
In general, the centrifugal casting process consists of selecting a female rotary mold formed with an elongated cavity having a network of inwardly opening grooves defining a predetermined stent configuration. A charge of casting material, usually in liquid form, is introduced into the cavity and the mold is rotated about a rotational axis to distribute the casting material throughout the mold. The rotational velocity of the mold may be increased to provide greater centrifugal force acting radially outwardly to press the casting material into grooves which define the structure of the stent. The casting material is then allowed to solidify forming a cast stent having a predetermined configuration.
The interior surface of the female mold is formed with a plurality of circumferential grooves and interconnecting channels. For example, each circumferential groove can have a continuous undulating pattern formed from a plurality of U-shaped pathways linked together in a consecutive alternating inverted relationship to provide a generally serpentine configuration. Interconnecting channels extend longitudinally between adjacent circumferential grooves connecting them together. Thus, the grooves and channels cooperate, when substantially filled with casting material, to provide a casting which provides a predetermined stent structure.
A charge of casting material preferably is introduced into the mold cavity at a predetermined rate to spread the material over the entire mold length in one continuous flow. It should be appreciated that the casting material may be introduced into the mold cavity while the mold is stationary or when it is rotating.
When the charge of casting material is introduced into the mold cavity and the mold is rotated, frictional forces develop between the casting material and the surface of the mold. Thus, the casting material is rotationally accelerated as it fills the grooves and channels provided in the mold surface. As the casting material fills the mold, centrifugal force from the rotating mold provides a pressure gradient acting radially across the thickness of the casting.
During the centrifugal casting process, the female mold is rotated at a sufficient tangential velocity to impart some centrifugal acceleration to the casting material. The centrifugal acceleration prevents slippage between newly introduced casting material and either the rotating mold surface or a previously deposited layer of material. In addition, centrifugal force helps prevent the casting material from falling out of the mold as it passes over the top arcuate section of the mold cavity.
After the charge of casting material has substantially filled the mold, the rotational speed of the mold may be increased while the casting material is allowed to solidify. By increasing the rotational speed of the mold during solidification, greater centrifugal force is applied to the casting material, making it possible to produce dense castings of high quality. It will be appreciated that the favorable thermal gradient and the radially outward acting centrifugal force produced by the rotating mold influence the solidification of the casting. As a result, porosity in the material sometimes occurs during the solidification of a casting material can be eliminated. However, if desired, a stent made in accordance with the present invention could be made porous to allow a therapeutic drug to be added to the surface or into the casting itself to produce a suitable drug delivery stent.
Rotational molding is another method for producing a stress-free stent having intricate strut patterns. Some of the steps for manufacturing a rotationally molded stent are very similar to the centrifugal casting process. The rotational molding process includes selecting a female rotary mold formed with an elongated cavity having a network of inwardly open grooves defining a predetermined stent configuration. However, the rotational molding process utilizes lower speeds than the centrifugal casting process. Normally, in rotational molding, the speed is in a range of about 5 to 20 rpms. The casting material for the rotational molding process is usually a dry powder, as opposed to a liquid, which is usually used in the centrifugal casting process. Therefore, as opposed to the centrifugal casting process, the casting material remains at the bottom of the rotating cavity of the mold, due to the force of gravity. As a result, in a rotational molding, the casting material stays in loose powder form until the surface of the mold reaches a temperature level that is high enough for the particles of the casting material to begin to adhere or sinter to the mold or each other at the layer nearest to the heated surface of the mold. Additionally, the female rotary mold is pre-charged cold with cold powder resin acting as the casting material. As the mold cavity is rotated, it is also heated to reach the temperature necessary to begin the sintering process. The heat source is eventually removed after a predetermined length of time and the mold is allowed to cool to solidify the casting material. As the material solidifies, a uniform melt structure is achieved.
A stent made in conjunction with the rotational molding process results in a stress-free structure which, as with the centrifugal casting process, can be formed in various complicated strut patterns. The precision of the stent will be determined by the precision in which the female rotary mold is formed. Stents made in conjunction with the present invention can be made from polymeric materials, including thermal plastic and thermal set polymers, and other biocompatible materials such as metal alloys including, but not limited to, tantalum, NiTi, as well as stainless steel 316L. The present process can be used to create stents of virtually any design.
The above and other objects and advantages of this invention will be apparent from the following more detailed description when taken in conjunction with the accompanying drawings of exemplary embodiments.